The phosphorylation of protein tyrosine residues is closely linked to cell growth and transformation. Methodological obstacles have been overcome which allow the measurement of protein tryosine kinase and protein phosphotyrosine phosphatase in cells. The development of a straightforward assay procedure for protein tyrosine kinase activity involves isolation of radioactive Tyr-P by Dowex 50 chromatography after base destruction of Ser-P and Thr-P residues in trichloroacetic acid precipitates of reaction mixtures using carbon-labeled Tyr-P as an internal standard. The assay is extremely sensitive enabling the quantification of a few hundred cpm of radioactive Tyr-P. The assay has been used to quantify protein tyrosine kinase activity in normal and malignant cells. The characteristics of the EAT cell protein tyrosine kinase are: two subcellular fractions of the kinase activity exist. The major form is membranous and detergent soluble. Both tubulin and casein can be used as exogenous substrates; tubulin is preferred. The activity is Mg(II) and Mn(II)-dependent. The expression of the cytosolic activity requires high Mn(II) and high pH; these conditions can be correlated with inhibition of PTPase activity. The kinase activity appears to be 1% of the PTPase activity in cell extracts. The kinase activity copurifies with the major form of PTPas activity on DEAE and is very thermolabile; at 37 C the half-life is about 6 minutes. Assay of protein phosphotyrosine phosphatase activity (PTPase) has been carried out in normal and cancerous cells using radioactive phosphotyrosine glutamine synthetase. Characteristics of the PTPase activity in Ehrlich Ascities Tumor (EAT) cells are: the activity is predominantly cytosolic and dilution-dependent with maximal activity about 5 nmol/min./mg. Heat stable nondialyzable inhibitors of the PTPase are found in boiled cell extracts which could act as regulatory molecules. PTPase elutes from DEAE columns in one major and another minor fraction neither of which is coincident with protein phosphoserine phosphatase activities (PSPase). The major form of the PTPase prefers protein substrates and readily dephosphorylates vinculin, phosphorylated by the Rous transforming kinase.